On October 23 1983, two explosive-packed trucks driven by suicide bombers rammed the barracks of American and French troops stationed in Beirut. 299 servicemen were killed – 241 of them Americans. In the aftermath of the attack, Ronald Reagan pledged to keep a military force in Lebanon. On February 7 of the following year, he ordered it to begin withdrawal.

There is a moral in the tale – or a historical reminder, at any rate. Reagan is remembered as a Cold War warrior, and a victorious one, but he fought shy of committing ground troops to America’s proxy wars abroad against the Soviet Union. He funded propaganda and backed dissidents in Eastern Europe; supported Solidarity in Poland; advised and armed the Contras in Nicaragua, and built up America’s armed forces – helping to heighten the economic pressure that eventually brought the Soviet system down. But he was seldom willing to put American lives on the line, as the story of this encounter with Islamic fanaticism, in its Iranian-backed Shi’ite form, helps to show.

Today’s struggle against Islamist extremism is in some senses not so very dissimilar from the one that Reagan and Margaret Thatcher fought against Soviet communism. Like that system, Islamism is an ideology – the bastard child of where the Wahabi strain of Islam and a modern conception of the state meet. Like it again, it is incompatible with liberal democracy, and the prosperity and liberty which both help to produce (and in a striking form, too, since stifling the opportunities and freedom of half the population of any country – i.e: women – is part of its make-up). Like it yet again, it is international, and has a presence in Britain.

There are also differences. The closest state to date to an Islamist one, Saudi Arabia, is nominally an ally. Iran, now a backer of Hamas once again, is strictly speaking not an Islamist country at all, since the religious ideology that underpins its governance is a form of Shi’ite and not Sunni Islam. In other words, Islamist extremism isn’t concentrated in a single state, as Soviet communism was. ISIS, which claims a caliphate to replace that which fell with the Ottoman empire, controls only part of one. Al Qaeda never ran a state – a fact that led to the decision by the Bush administration to treat its captured operatives and others outside the terms of the Geneva convention.

Barack Obama, David Cameron and other western leaders must thus grapple with difficulties that Reagan and Thatcher never faced. Yesterday, so to speak, the main security threat to western interests was Al Qaeda, and the possibility that it would run riot in Yemen. Today, it is ISIS in Iraq – the western horror at whose atrocities is inflamed by its treatment of Christians, which Luke de Pulford writes about on this site today. The enemy is protean. Furthermore, Obama and the rest of them must face runs of opinion that are amplified by social media. What one moment is a surge of opinion for the prevention of genocide turns the next into a swell for the withdrawal of troops.

For all these problems and differences, though, today’s leaders can learn from yesterday’s. Iraq and Afghanistan and our debts and deficit have taught us that boots on the ground don’t turn conflicted states into liberal democracies – and that attempts to prove the contrary cost more in blood and treasure than we can afford. Islamism cannot be occupied out of existence. But intervention and ground troops aren’t the same thing: indeed, it’s possible to intervene without sending them in at all – in the same ways that Reagan deployed. Allies can be advised, trained, armed; special forces can sometimes be sent in.

Our recent poll of party members above found limited support for some of these measures – almost a third were happy to train Iraq’s military; a fifth of respondents were willing to send in special forces; two thirds did not say that we should steer clear of Iraq altogether. Specifically, a quarter backed air strikes. The outlook of members of political parties shouldn’t be confused with that of voters more widely. But there is no reason to think that this centre of gravity isn’t that of the rest of the public. It’s true that politicians have a responsibility to lead public opinion, and not simply follow it. But these chary instincts are justified, as they were when the Commons revolted last year against air strikes on Syria.

The Government is suggesting this morning that such strikes may happen if the slaughter toll rises in Iraq. Perhaps they should. But some hard questions would have to be asked. What if they failed? Would ground troops be sent in, and Britain be re-immersed in Iraq? Or would the West back off, leaving itself humiliated and ISIS triumphant? What if Putin moves in Ukraine in the meantime? What’s Plan B? These are questions that Parliament must be recalled to debate if the Government wants to make a decision before September. And Ministers must always, always remember that the most urgent challenge to our interests from Islamist extremism is right here at home.